Filtering Contact Discharges - Part 2, High Pass Filtering(for troubleshooting or simulation of ESD from small objects)

Figure 1. ESD Simulator Fitted With High Pass Filter

Abstract: High pass filtering of an ESD
contact discharge can be used for troubleshooting ESD problems
with equipment or simulating ESD from small metal objects in a
repeatable way. A method is described for constructing a simple coaxial
capacitor to provide a high pass filter that leaves just the initial
peak of the hand-metal ESD waveform while attenuating the low frequency
components.

Discussion: Last month's Technical Tidbit for September 2006 described how to low pass filter the contact discharge ESD current from an IEC 61000-4-2ESD
simulator to approximate a high voltage air discharge ESD event as well
as for
troubleshooting designs. This month, high pass filtering of the IEC
contact discharge ESD current is discussed both for troubleshooting as
well as simulating a
class of ESD events as before. In this case, the high pass filter
leaves just the initial spike of the contact discharge by attenuating
the
lower frequency components that comprise the "body wave" that lasts for
several tens of nanoseconds after the initial spike.

Figure 1 shows an ESD simulator with the finished filter installed. The visible copper foil
is in fact one of the plates of a coaxial capacitor formed by the air
discharge tip of the simulator and the copper foil. Figure 2 shows a
short length of plastic tubing with a small PVC cap plugging the end.
The PVC plug is in fact a vacuum port cover for an automobile intake
manifold obtained from an auto parts store. The tube and plug form
the dielectric of our coaxial capacitor.

Figure 2. Plastic Tube and PVC Cap Dielectric for Coaxial Capacitor

Figure 3 shows the dielectric assembly installed over the air discharge
tip of the ESD simulator. In Figure 4, the copper foil tape is added. The
capacitance between the ESD simulator tip and the copper foil couples
only the initial spike of the ESD current waveform to the intended
target.

Figure 4. Tip With Insulation Wrapped in Copper Foil to Form a Capacitor

The capacitor composed of the the simulator tip and copper foil is
charged by the simulator with each ESD event. The specific simulator
used here, the KeyTek MiniZap,
has a leakage path after the discharge that will discharge the tip-foil
capacitor as long as the foil has a conductive path back to the simulator ground connection. One needs to provide a few
seconds to allow this to happen. Figure 5 illustrates using a static
meter to see how fast the charge is drained from the simulator tip
after a contact discharge with the tip floating. For
the MiniZap, two or three seconds is more than adequate. One can check
to see if enough time is allowed between discharges by measuring the
discharge current into a grounded target. If the charge on the tip-foil
capacitor is not drained between discharges, the
discharge current will be reduced. Allow enough time between discharges
to get the maximum current. Some ESD simulator designs may require a
high valued high voltage resistor be built into the tip-foil capacitor
to discharge it between ESD events.

Figure 5. Checking Static Charge on the Simulator Tip

Figure 6 shows a ~1 kV contact discharge of the MiniZap ESD simulator fitted with the high pass filter through a Fischer Custom Communications F-65
current probe into a ground plane
to measure the discharge current waveform. The MiniZap shown is not
fitted
with its IEC tip so the rising edge of the initial current spike will be
somewhat faster than the 700 ps called for in the IEC standard. In this
case, the current risetime is about 300 to 400 picoseconds. Figure 7
shows the resulting current waveform. The current has a peak of about three
Amperes and a duration of a little less than two nanoseconds.
Only 1 kV was used to keep the
waveform on the scope screen without having to use external attenuators
on the scope. The waveform at 8 kV will be the same, just eight times
larger.

Contrast the current waveform in Figure 7 using the high pass filter to
that in Figure 8 with the filter removed. In Figure 8, from last
month's Technical Tidbit, one can see not only the initial spike, but
also the much longer duration body wave that follows.

By filtering out the low or high frequency components of a contact
discharge ESD current, useful information can be obtained for
troubleshooting
equipment problems. If the problem is due to the fast spike, different
mechanisms are likely to be at play than if the problem is caused by
the lower frequency components of the waveform. Also high pass
filtering of the ESD current can be used to simulate ESD events from
relatively small pieces of
metal in a repeatable way. In some ways, the current waveform of a high
pass filtered contact discharge is like a Charged Device Model(~260K pdf file)test.

Summary:
A method of high pass filtering an ESD simulator output is
described using an easy to build coaxial capacitor. The resulting short
pulse of current can be used for troubleshooting circuits and systems
as well as to simulate ESD events from relatively small pieces of
metal. The current pulse is in some ways similar to the current generated in a Charged Device Model
ESD event.

If you like the information in this article and others on this website, much more information is available in my courses. Click here
to see a listing of upcoming courses on design, measurement, and
troubleshooting of chips, circuits, and systems.

Click herefor a description of my latest seminar to be available in the Fall titled:

EMC
Lab Techniques for Designers
(How to find EMC problems and have some
confidence your system will pass EMC testing while it is still in your
lab).

Check out my podcast containing mp3 format short tutorials, tech news and more! Just click on the microphone to see the listing
of free
audio programs. Content is added every week on technical topics so check back frequently.